SYSTEM PRESSURE CONTROL
TECHNICAL BULLETIN
The EXTROL® Principle
EXTROL expansion tanks provide a pneumatic cushion
capable of continued operation without the loss of air
into the system through absorption. This is possible
because a flexible diaphragm (or bladder) separates
the water and air in the tank. The permanently sealedin air cushion can be pre-charged to minimum system
pressure to permit the smallest tank possible.
The L and LBC series are ASME certified and are available
in total tank volumes from 10 to 3,963 gallons. They have
integral floor stands and are easily installed.
Maximum Working Pressures: Up to 300 PSIG
Maximum Operating Temperature: 240°F
Flexible Bladder
Precharged Air Cushion
Inner Water Reservoir
The AX Series is ASME certified and comes in two styles,
vertical and horizontal. Installation can be suspended or freestanding configurations. Available in total tank volumes from
8 to 211 gallons.
Maximum Working Pressure: Up to 300 PSIG
Maximum Operating Temperature: 240°F
Temperature and Pressure Effects in
Hydronic Heating and Cooling Systems
A system is designed to operate within a certain temperature
band. One limit is set by the ambient temperature and the other
is determined by the function of the system. For example, the
temperature is reduced in a chilled water system and increased
in a heating system.
Minimum
System Temp.
Maximum
System Temp.
Heating
Ambient
Average
Design Temp.
Chilled Water
Average
Design Temp.
Ambient
System Type
The volume of water within a system varies with the temperature.
At any point above 39.2°F, an increase in temperature results
in an increase in volume (see Figure 1). Therefore, as the
temperature of a system changes within the operating band, a
change in volume must be accommodated.
Pressure
Pressure in the system will fluctuate during the operating cycle.
It is the function of the expansion tank to prevent an increase
in pressure to a level where safety relief valves may operate.
Knowledge of the pressure range occurring at the expansion
tank is essential in sizing an EXTROL.
Minimum Pressure at EXTROL
Minimum
Operating Pressure
=
Static
Pressure
+
®
Pressure to
Prevent Boiling
i) S
tatic pressure – is the pressure exerted by a column of
water. To calculate, estimate height of system in feet above
the EXTROL and multiply by 0.433 to give PSIG.
ii) Pressure to prevent boiling – water boils at 212°F in normal
atmospheric pressure, but as the pressure increases, the
boiling point also rises. Therefore, if a system is to operate
over 212°F without boiling, the pressure must be increased
over the static level. The increase necessary for various
temperatures is shown in Table 1.
Figure 1 - Net Expansion of Water
1.06
1.05
SPECIFIC VOLUME
Temperature Affects Volume
1.04
1.03
1.02
1.01
1.00
39.2
60
80
100
120
140
160
TEMPERATURE
180
200
220
240
Table 1 - Pressure to Prevent Boiling
Maximum Operating
Temperature in °F
Approximate Positive Pressure Required
200°
2 PSIG
205°
3 PSIG
210°
5 PSIG
215°
7 PSIG
220°
9 PSIG
225°
11 PSIG
230°
13 PSIG
235°
16 PSIG
240°
19 PSIG
Figure 2 - Analyzing the Critical Points In System
Maximum Pressure at EXTROL®
The upper pressure limit at which the tank will operate is
the sum of the minimum pressure plus the system allowable
pressure increase.
Maximum
Operating Pressure
=
Minimum
Operating Pressure
+
Allowable Pressure
Increase for System
5 PSIG - Adequate Positive Pressure
50' = 22 PSIG
Static
Pressure
Pressure Change for
Expansion = 30 PSI
100' = 43 PSIG
Static
Pressure
Relief Setting … 80 PSIG
Pump Effect ... 15 PSIG
After evaluating the pressure change for expansion at all critical
points in the system, the smallest one is chosen as the allowable
pressure increase for the system. (See Figure 2).
Example – evaluating pressure change for expansion at the boiler:
Pump Head
20 PSI
Relief Setting … 100 PSIG
Pump Effect ... 20 PSIG
Relief valve setting at boiler........................................... 100 PSIG
Minus 10 percent weepage protection............................ -10 PSIG
Pressure Change
for Expansion
at Boiler = 22 PSIG
Equals maximum allowable pressure at boiler................. 90 PSIG
Minus minimum pressure at boiler.................................. -48 PSIG
Maximum Allowable
Pressure Increase at
Tank Can Be No More
Than 22 PSI
Equals permissible pressure change at boiler................. 42 PSIG
Minus change due to pump operation............................. -20 PSIG
Equals pressure change for expansion at boiler.............. 22 PSIG
Pump Effect
If the EXTROL tank is downstream and the pump is upstream
of the component, the pressure will increase when the pump
operates. If the reverse is true, pressure will decrease when the
pump is running. This is because the hydro-pneumatic tank is
the only component that is unaffected by pump operation. The
only time pressure at the tank varies is when system temperature
changes and expanded water flows to or from the EXTROL, thus
changing the volume of the captive air cushion.
The point at which the EXTROL is connected is an important
consideration. By locating it at pump suction, it is assured that
all pump pressure effects are positive and vacuums cannot be
drawn. EXTROL connection at any other point is feasible and
occasionally may be desirable, but the pump pressure effects
should be considered.
Figure 3 - System and Pump Characteristics
30
PRESSURE - FT H2O
The pressure developed by the system pump is determined by
the pipe network to which it is connected. Every system has a
unique operating characteristic band, often simplified to a curve
on a graph (See Figure 3). The point where system and pump
characteristics intercept defines the point of pump operation.
The pressure developed by the system pump is dissipated
in overcoming friction as fluid flows through the system. The
amount of this pressure change at any given component varies,
depending on its proximity to the point of tank connection.
PUMP CURVE
25
SYSTEM
OPERATING
ENVELOPE
20
15
DESIGN
OPERATING
POINT
10
SYSTEM CURVE
5
0
0
10
20
30
40
50
SPECIFIC FLOW - G.P.M.
60
Typical Specification Pressurization
and Air Elimination System
Typical Air Elimination System
Top Floor
Riser To
Upper
Floors
The tank shall be supported by steel legs or a base (integral ring
mount) for a vertical installation. Each tank will have a heavy duty
replaceable butyl bladder.
P
P
Deaeration Valve
and Pressure
Gauge
Air Seperator
& Eliminator
Air Seperator & Eliminator
Location When
Location When Pressure Drop
Pressure Drop
Thru Lateral is Low
Thru Lateral is High
Pressure
Gauge
P
Pump
Pressure
Gauge
P
Additional
Pumps
Pressure
Gauge
P
Automatic
Fill Valve
Temperature
Gauge
T
Air Seperator
& Eliminator
(Recommended)
Water
Meter
Drain
Valve
Boiler
The manufacturer shall be AMTROL Inc. The manufacturer will have
at least five years experience in the fabrication of bladder-type ASME
expansion tanks.
3. Air Separator (Tangential Type): All free air originally contained in
the system, and all entrained air bubbles carried by system water shall
be eliminated at all system points as indicated in the drawings.
The air separator shall be cast iron or welded steel, constructed, tested
and stamped in accordance with Section VIII, Division 1 of the ASME
Code for a working pressure of (150) (___) PSIG as manufactured by
AMTROL Inc.
The pressure drop through the air separator at the specified flow rate
shall be as shown on the drawings.
Comprising
Connection of EXTROL to System
1. AX Series Expansion Tank (Diaphragm type pre-pressurized):
Unlike a conventional tank, the permanently sealed air cushion does not
The pressurization system shall include an EXTROL® diaphragm-type
expansion tank which will accommodate the expanded water of the
system generated within the normal operating temperature range,
limiting this pressure increase at those components in the system
to the maximum allowable pressure at those components. It shall
maintain minimum operating pressure necessary to eliminate all air.
The only air in the system shall be the permanent sealed-in air cushion
contained in the diaphragm-type tank, Model No. ____. Dimensions
shall be as indicated on the drawings.
The expansion tank shall be welded steel, constructed, tested and
stamped in accordance with Section VIII, Division 1 of the ASME Code
for a working pressure of (125) (___)PSIG and air pre-charged.
The tank shall be supported by steel legs or a base (integral ring
mount) for a vertical installation or steel saddles for a horizontal
installation. Each tank will have a heavy duty butyl diaphragm.
The manufacturer shall be AMTROL Inc. The manufacturer will have at
least five years experience in the fabrication of diaphragm-type ASME
expansion tanks.
2. L
& LBC Series Expansion Tank (replaceable bladder-type prepressuriezed): The pressurization system shall include an EXTROL
replaceable bladder-type expansion tank which will accommodate
the expanded water of the system generated within the normal
operating temperature range, limiting this pressure increase at those
components in the system to the maximum allowable pressure at
those components. It shall maintain minimum operating pressure
necessary to eliminate all air. The only air in the system shall be the
permanent sealed-in air cushion contained in the replaceable bladdertype tank, Model No. ____. Dimensions shall be as indicated on the
drawings. The expansion tank shall be welded steel construction,
tested and stamped in accordance with Section VIII, Division 1 of the
ASME Code for a working pressure of (125) (175) (250) (___)PSIG
and air pre-charged.
require replenishment so DO NOT connect it to a separator or other “air
control” device.
As with any expansion tank, it is recommended that the EXTROL is
connected to the suction side of the pump. This will prevent the pump
head from becoming subtractive which may result in entry of air and
vapor formation at high points in the system.
By connecting the tank on the boiler return, displacement of the coolest
system water into the tank is ensured, thus reducing energy wastage.
Low pressure drop components, such as separators and boilers, may be
located between the point of connection and pump suction if desired. The
design of the system will dictate the feasibility of this.
Natural convection losses and the entry of air into the tank are eliminated
by connecting to the side or underside of the main pipe. Any potential air
traps in the piping connecting the tank to the system should be vented.
Typical Connection Diagram
To Air Separator
& Pump Suction
Pressure Gauge
I
To Boiler
Strainer
Lock Shield
Gate Valve
Pressure Reducing Valve
EXTROL
Diaphram/
Bladder Type
Prepressurized
Expansion Tank
1400 Division Road, West Warwick, RI USA 02893
T: 800.426.8765 F: 800.293.1519
www.amtrol.com
© 2014 AMTROL Inc.
Reduced Pressure Zone
Type Back
Flow Preventer
Meter
Mark of the
Originator
MC# 1140 Part #: 9017-100 (01/14)